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Part III. — Eighteenth Annual Report 



This size-limit of 17 inches is, therefore, the upper limit, or nearly so, 

 at which immature plaice — i.e., those which have never spawned — are 

 found in the North Sea. At 18 inches Holt considered that the propor- 

 tion of immature to mature was " infinitesimal." One specimen with 

 ovaries unripe was obtained at a greater size than 18 inches, but later in 

 the season, and it was probably a " spent," not immature, fish. 



The distinction between spent or spaivned fish and immature fish may 

 be referred to briefly before proceeding further. The immature fish are 

 those which have never spawned and show no signs of going to spawn. 

 If they show signs of going to spawn in the ensuing season, they may be 

 taken as ripe for the purposes of this paper. The spent or spawned fish 

 are those which have spawned at some time or other. Theoretically, the 

 distinction is great, but in practice it is at times exceedingly difficult to 

 discriminate between the two. According to Holt, the spent fish could 

 always be distinguished from the immature, either by the presence of 

 some remaining ripe ova, by the shrunken condition of the ovary, or 

 when the ovary had contracted to its previous condition of unripeness by 

 the groove that remains along the interspiuous rays where the developed 

 ovary had been. The two former characteristics always lead to a certain 

 diagnosis of the spent condition, but the last has been called in question 

 by Cunningham, and I have also found it unreliable. The groove or 

 hollow along the interspinous rays is present (to a certain extent) in the 

 young forms, which are evidently immature, and is, therefore, not 

 diagnostic of the spawned condition. In short, so far as I have been 

 able to determine up to the present, the ovary and the parts in 

 connection with the ovary return to exactly the same 

 condition in the spawned fish as they were previously in the 

 immature.* It should be mentioned that Cunningham imagined at 

 one time that he had discovered a sure sign of the spawned fish in small 

 opaque bodies which he found intermixed with the undeveloped ova. 

 These he thought were the degenerated remains of ova which had 

 matured to a certain extent, but had not been spawned with the rest, and 

 had then atrophied. He withdrew this distinction later, however, when 

 he found that these opaque bodies were present in ovaries of small fish 

 which were really immature. When the spawned fish is given time to 

 return to its condition previous to spawning, there are, therefore, so far 

 as known, no means of distinguishing the spent from the immature fish. 



During the spawning season — from December to May for the whole 

 North Sea — the difficulty of distinguishing between the spent and 

 immature conditions is not great except at the very beginning and the 

 end of the season. Doubtful cases certainly occurred in the course of 

 my own investigations between these times in the case of large fish with 

 unripe reproductive organs ; but their number was comparatively small, 

 and I have thought it better to consider them immature. At the 

 beginning of the season and earlier — during October, November, and the 

 first half of December — the difficulty was to tell whether a fish with 

 reproductive organs a little larger than the normal unripe condition was 

 really going to spawn that season or not. In consequence of this diffi- 

 culty I have been obliged to form a third class, and consider a number of 

 fish as " doubtfuls." These will be referred to later. At present it may 

 be said that the difficulty of distinguishing between a spent and immature 

 fish, and of telling whether a fish is going to spawn in the ensuing reason 

 or not, is greater for the males than for the females. 



Cunninghamt was the next naturalist who took up this matter in 

 England. He found that for the plaice of the English Channel 15in. 



* Petersen and Fulton {I.e.) were of the same opinion, 

 f Jour. Mar. Biol. Ass., Vol. III., 1893, p. 70. 



